A variety of techniques for generating depth maps and autofocusing on objects have been implemented in the past. One method conventionally used in autofocusing devices, such as video cameras, is referred to a hill-climbing method. The method performs focusing by extracting a high-frequency component from a video signal obtained by an image sensing device such as a CCD and driving a taking lens such that the mountain-like characteristic curve of this high-frequency component is a maximum. In another method of autofocusing, the detected intensity of blur width (the width of an edge portion of the object) of a video signal is extracted by a differentiation circuit.
A wide range of optical distance finding apparatus and processes are known. Such apparatus and processes may be characterized as cameras which record distance information that are often referred to as depth maps of three-dimensional spatial scenes. Some conventional two-dimensional range finding cameras record the brightness of objects illuminated by incident or reflected light. The range finding cameras record images and analyze the brightness of the two-dimensional image to determine its distance from the camera. These cameras and methods have significant drawbacks as they require controlled lighting conditions and high light intensity discrimination.
Another method involves measuring the error in focus, the focal gradient, and employs the focal gradient to estimate the depth. Such a method is disclosed in the paper entitled “A New Sense for Depth Field” by Alex P. Pentland published in the Proceedings of the International Joint Conference on Artificial Intelligence, August, 1985 and revised and republished without substantive change in July 1987 in IEEE Transactions on Pattern Analysis and Machine Intelligence, Volume PAMI-9, No. 4. Pentland discusses a method of depth-map recovery which uses a single image of a scene, containing edges that are step discontinuities in the focused image. This method requires the knowledge of the location of these edges, and this method cannot be used if there are no perfect step edges in the scene.
Other methods of determining distance are based on computing the Fourier transforms of two or more recorded images and then computing the ratio of these two Fourier transforms. Computing the two-dimensional Fourier transforms of recorded images is computationally very expensive which involves complex and costly hardware.
Other methods have been implemented by comparing multiple images to determine a depth. One method includes using an image that is in-focus and an image that is out-of-focus where the in-focus value is zero, hence the mathematics are very simple. Another method utilizes two separate images, with different focuses, where the distance between the images is the blur of the first image minus the blur of the second image. Computations are performed to determine the depth, although there are significant drawbacks with the past methods implemented in these computations.